A semiconductor memory is generally considered to be one of two types of memory—volatile memory or nonvolatile memory. Each of these types of memory has its limitations. For example, volatile memory technologies generally have relatively lower storage densities and faster access times, as compared to nonvolatile memory technologies. However, volatile memory technologies typically require a constant power source to retain the memory contents, unlike nonvolatile memory technologies, which do not generally require a constant power source.
Two conventional types of volatile memory are dynamic random access memory (“DRAM”) and static random access memory (“SRAM”). The relatively simple memory cell structures of DRAM allow for higher performances and higher storage densities. A memory cell in a DRAM cell usually consists of a transistor and a capacitor to store a bit of data. Since capacitors typically leak electrical charge, a DRAM needs to be constantly refreshed to retain the memory contents. Electrical power is typically consumed to refresh a number of memory cells (e.g., every memory cell) in a DRAM, even if the memory or part of the memory is not being used. By contrast, an SRAM cell usually includes several transistors implemented as a flip-flop to store each bit of memory. An SRAM cell is typically sensitive to power glitches, during which some or all of the memory contents are lost. Although an SRAM cell does not require refreshing, it commonly requires constant power to maintain its memory contents, even if the memory or part of the memory is not in use.
Conventional nonvolatile memory, such as Flash memory technology, typically uses complex logic, including state machines and other logic devices, to read and program a memory. Flash memory usually is slower than SRAM or DRAM since Flash memory includes a complex interface and requires erasing and/or programming in blocks of bits. However, Flash memory retains its contents when power is removed from (i.e., is not applied to) the memory.
Flash memory may be implemented as NOR and NAND Flash types. NOR Flash allows dynamic memory access (“DMA”) and, thus, is relatively faster than NAND Flash in accessing small amount of data. However, NOR Flash is slower than volatile memory, such as SRAM or DRAM. NAND Flash offers higher densities on a given die size compared with NOR Flash.
There are continuing efforts to improve non-volatile memory technology.
Although the previous Drawings depict various examples of the invention, the invention is not limited to those specific examples. Furthermore, the depictions are not necessarily to scale.